The present invention generally relates to plastic containers for beverages and, more particularly, to a blow molded plastic container having an improved base structure for use with non-carbonated and low carbonation beverages.
Prior to the development of integral base structures, blow molded containers were produced with a rounded or outwardly convex bottom over which was glued a separate base cap. The base cap had a contact ring for stabilizing and supporting the container. These base cap containers were unsatisfactory since, being of a two piece construction, they were costly.
With the wide acceptance of plastic beverage containers, and in an effort to reduce cost, containers having integral self-supporting base structures were developed. These plastic containers generally incorporated a bottom, forming an outer support ring having an upwardly extending and downwardly concave recessed center, often referred to as a "champagne bottom".
During the production cycle of a blow molded plastic container, a preform is axially stretched and inflated to impart both axial and radial elongation to the material. In the art, this forming is known as biaxial orientation or elongation. Early integral base structures were vulnerable to stress cracking and crystallinity problems as a result of over stretching and poor temperature control during molding. Specifically, the center region of the base structure received little stretch and was formed of amorphous non-oriented material, while the outer edges of the base were overstretched and thin. Both resulting regions were weak.
In the current production of plastic containers, the weakest part of the container continues to be the base structure. For example, when a container formed with a champagne bottom is filled with a carbonated beverage, the tendency of the champagne bottom is to invert to a downwardly convex shape. Various configurations have been developed to provide reinforcement to the bottom structure in an attempt to prevent this inversion.
To overcome these limitations, containers have been provided with reinforced base structures including ribs or webs of increased thickness. Unfortunately, these reinforced structures increased the amount of raw material needed to produce the final product an correspondingly increased the final cost of the container.
One container with a self-supporting and reinforced base is disclosed in U.S. Pat. No. 4,334,627. In this patent, a container is disclosed as having a base structure with a plurality of internally formed and radially extending reinforcement ribs. In addition to the increased raw material requirements, the initial formation of the ribbed preform is time consuming and further adds to the overall cost of the container.
Another container with a self-supporting base is disclosed in U.S. Pat. No. 4,892,205. This patent discloses forming a container having circumferentially spaced hollow feet which exhibit structural integrity sufficient to resist roll out. The feet have a flat bottom surface of a generally trapezoidal shape which decreases in width along a radially inward direction. The footed base structure has a contact surface which extends less than 360.degree. base structure. Because of this, the upright stability of the container may be compromised when placed on a rack or grid shelf of the type often found in a household refrigerator.
While containers of the above-mentioned type work satisfactorily with highly carbonated beverages, e.g. beverages such as soft drinks, when used with non-carbonated or low carbonation beverages, the reinforcement structures of these containers provide strength greater than that required and the intricate molding requirements are more complex than necessary resulting in an more costly container. Low carbonation, as used in the present invention, generally refers to carbonated beverages having internal container pressures of about forty-five pounds per square inch (45 psi) and less.
As can be seen from the above discussion, there is a need for a container having a base structure strong enough to resist roll out, when used for packaging low carbonation beverages, yet which may be cost effectively produced so as to permit its use with non-carbonated beverages. The present invention is directed to fill this need.
With the above in mind, it is an object of this invention to provide a container which can be used with both non-carbonated and low carbonation beverages. In achieving this, the container is inexpensive to produce and has a base configuration whose simplicity lends itself to processability.
An additional object of the invention is to provide the container with a base structure that exhibits increased stability on grid type support surfaces.
In satisfying the above objects, the present invention provides for a blow molded plastic container which may be readily produced from a preform without the structural requirement of internal ribs. Because of its simplicity, the base structure of the present invention allows for a container to be constructed without precision technology in either the processing of the plastic, the forming of the preform or the actual blow molding of the final container.
Accordingly, a plastic container embodying the principles of the present invention includes a base structure having a continuous bearing surface for contacting a support surface. The continuous bearing surface provides increased stability to the container when stored or positioned upright on a grid or rack type shelf. The bearing surface is formed by the radius where the outer wall merges with the inner wall of the base structure. The radius of the bearing surface is kept as small and as "sharp" as possible. Roll out, when the container is used with low carbonated beverages, is prevented by providing the dome shaped, downwardly concave inner wall of the base structure with a plurality of panels.
The panels are positioned adjacently around the outermost portion of the inner wall. Each panel is inclined relative to the dome of the inner wall so as to form corners, at the juncture therewith, directed interiorly of the container. The corners and the panels cooperate to stiffen the inner wall and enable the base structure to maintain its structural integrity when the container is filled with a beverage. Adjacent to the bearing surface, the lower edge portions of the panels themselves are substantially straight. The upper edge portions of the panels are generally arcuate at the juncture with the inner wall and, thus, the corners are also arcuate. The bearing surface is part of a support wall that connects the inner wall to a convex outer wall. The outer wall merges the base structure with the side wall of the container.
As the dome is forced downward by the internal pressure of the container or the weight of the contents, the lower edges of the panels interact with the radius of the bearing surface and the corners. In trying to invert the corners, the interaction creates a "pinch-point" at the radius of the bearing surface which further increases stiffness and resistance to roll out.
It is important that the panels be planar or if not perfectly planar, convex or bowed toward the outer wall so that downward forces on the inner wall tend to pinch the panels toward the outer wall and thereby preclude roll out of the inner wall.
Additional advantages of the invention will be apparent to those skilled in the art from the following description of the preferred embodiments and the appended claims.